The measurement data from NDT/NDI devices used for the routine monitoring of structural integrity must be of sufficient accuracy to allow a valid assessment to be made of the condition of the structure under test. Examples of such structures are pipes and vessels widely used in the petrochemical and other industries. Examples of measurement data are pipe wall thickness and other geometric conditions, including, but not limited to, the presence of irregular surfaces (e.g. corrosion, oxide, etc.) and flaws (e.g. porosity, cracks, etc.).
The decision to perform or not perform maintenance on a structure is made based on the assessment of the measurement data. Therefore, the measurement accuracy will have a direct impact on the decision. The consequence of inaccurate measurement data that underestimates an unfavorable condition of a structure can result in failures occurring before maintenance is performed. Conversely, inaccurate measurement data that overestimates an unfavorable condition of a structure can result in performing expensive and unnecessary maintenance.
One of the most common NDT/NDI devices used for assessing structural integrity is a corrosion gage, such as the instant assignee's 37 DLP product. Products of this type typically employ a ‘dual-element’ probe or probe system that contains one element for acoustic transmission and another for acoustic reception, preferably packaged in an integral housing. The two elements are set at a fixed angle, thereby setting a fixed focal depth and ‘V-Path’ within the object being tested. Although this element positioning provides advantages for measuring corrosion wear, measurement errors, known as ‘V-Path errors’, can be introduced when measuring thicknesses at depths other than that of the focal depth.
The specific challenge herein dealt with is to provide a method that will ameliorate the measurement errors resulting from V-Path echo, which is the energy path traveled by the acoustic wave after the energy is transmitted into the target material and reflected from the back-wall of the material and into the receive element of the transducer. Particularly, V-Path errors occur when thickness measurements are being made on a material thinner than the focal thickness of the transducer.
Existing efforts have been made to eliminate or reduce such errors as described above. Thus, embodiments employing pre-defined data for the V-Path, or time distortion, correction in the calculation of a thickness measurement are well known by those skilled, and are therefore not described in detail herein.
One conventional solution for V-Path error compensation employs pre-defined static data tables to compensate for the time distortion; however, this solution has the drawback of not accounting for actual material sound velocity, transducer wear and manufacturing variances in transducer population.
Materials under inspection have their own individual velocities denoted as V, where V=material velocity. U.S. Pat. No. 3,554,013 teaches a hardware error correction circuit for ultrasonic thickness gauges. It is not a software method and presents the drawbacks of thermal and other electronic drift and material costs.
U.S. Pat. No. 4,570,486 teaches V-Path calibration for UT thickness measurement using hardware error correction circuits for ultrasonic thickness gauges. It is not a software method and presents the drawbacks of thermal and other electronic drift and material costs.
Current V-Path methods using a pre-determined data table, called “V-Path Table,” use empirical methods of deriving data to generate the Table. The predetermined Table is generated by using TOF measurement methods on a batch of typical transducers of one model. It is then used for hundreds of the transducers of the same model for many years. The existing V-Path Table is herein referred to as the “Empirical V-Path Table”.
Using an Empirical V-Path Table to compensate all the transducers of one model is less accurate because of variations of transducer factors such as acoustical focal depth and saturation of the acoustic barrier. The factors causing such variations include manufacturing tolerance changes in different batches of transducers, changes in material characteristics, and changes caused by wear-and-tear.
Accordingly, a solution that overcomes the drawbacks described above and results in advantages highly valued by potentially affected industrial and public infrastructure concerns, needs to:                a. Improve measurement accuracy;        b. Extend the longevity of transducers along with their measurement accuracy; and        c. Improve measurement accuracy of generic transducers for which the pre-defined V-Path data is unknown.        